In circumstances where it is desired to separate materials into two size groups, such as the separation of solid material from liquid material, usually the materials are merely fed to a machine having a screen, which can be vibrated, to effect the desired separation. In such circumstances the materials are usually merely piped to and deposited at the center of the screen, in the case of a circular screen wherein the solids are discharged at the periphery, or, in the case of a rectangular inclined screen, the materials are deposited at the top thereof with the solids being allowed to fall off the screen at the bottom. This type of separation results in a very inefficient use of screen area since only a small portion of the screen is actually performing the separation.
One attempt at more efficient screening is disclosed in U.S. Pat. No. 4,065,382. There the feed is split into a plurality of feeds to be fed to parallel tilted screens. In an attempt to universally distribute the feeds over the screens, the material is splashed onto a flange. However, such action does not guarantee an even distribution across the entire width of the screens.
Another problem involved with the aforementioned screening operations is that because the screens are usually of a very fine mesh to permit only liquids to pass therethrough, they wear out very easily. As a solution to this problem, it has been proposed to provide a screen package which consists of the necessary fine mesh screen and a heavier back-up screen of larger mesh. The two screens are tensioned together and the back-up screen provides the fine mesh screen with additional support for longer life. While such a design is satisfactory for this purpose, the fine mesh screen does eventually wear out and then the entire screen package must be replaced including the costly back-up screen, even though it has not at all worn. Furthermore, even tensioning of both screens together is not possible because while the back-up screen may be properly tensioned, the finer screen will tend to be looser than desired.